Objective: To determine the diagnostic accuracy of the bedside index for severity in acute pancreatitis in comparison with Ranson scores in predicting mortalities and severities in patients with acute pancreatitis.Methods: The cross-sectional study was conducted at the Department of Emergency Medicine, Aga Khan University Hospital, Karachi, from July 1, 2017, to January 1, 2018, and comprised patients who presented with acute pancreatitis. The bedside index for severity in acute pancreatitis score was applied in the emergency department and the patients were followed up in ward/intensive care unit where Ranson scores were calculated within the following 48 hours. Both the scores were calculated and compared for the prediction of severity and mortality for each patient. Data was analysed using SPSS 20.Results: Of the 136 patients, 88(64.7%) were males and 48(35.3%) were females. The overall mean age was 42.04±16.42 years (16-75 years), On the basis of two scores, mild and moderate acute pancreatitis was diagnosed in 123(90.4%) and 119(87.5%) patients respectively, while severe condition was diagnosed in 13(9.6%) and 17(12.5%) patients respectively. The bedside index had specificity 94.62% compared to 91.54% for Ranson score; sensitivity 100% vs 100%; negative predictive value 100% vs 100%; positive predictive value 46.15% vs 35.29%; and diagnostic accuracy 94.85% vs 91.91%.Conclusions: The bedside index for severity in acute pancreatitis and Ranson score were both found to be reliable tools in predicting mortalities and severities in patients with acute pancreatitis